by
B. A. Ola-Adams
Forestry Research Institute of Nigeria
Ibadan
INTRODUCTION
There are several publications which stress the urgent need for conservation of the genetic resources of species and ecosystems in general (UNEP 1975, Richardson 1970, Hedberg & Hedberg 1968, UNESCO 1973) and of Nigeria in particular (Charter 1968, Roche 1973, Okali 1975, Ola-Adams & Iyamabo, 1977).
Economic development with the concomitant increases in population, rising standards of living and cash-cropping, is resulting in large-scale destruction of natural ecosystems.
Baykal (1975) estimated that the consumption of sawnwood in Nigeria had reached about 1 000 000 m3 in 1975 and suggested that the rate of increase in demand for sawnwood would likely remain high for another 5 to 10 years at 8 to 10 percent per annum. Intensive logging is likely to increase with the rapid growth of the home market. In order to meet the increasing demand, large areas of the natural forests are being cleared for the establishment of fast growing indigenous and exotic tree species. In this paper the implications of this trend for forest conservation are examined.
PRESENT SIGNIFICANCE OF VEGETATION ZONES IN FORESTRY
Occupying a narrow strip along Nigeria's coast are the mangrove forests and other coastal vegetation types. These cover an area of 12 782 km2 (522 km2 reserved) and contain mainly Rhizophora species (R. racemosa 1, R. mangle and R. harrisonii) and Avicennia africana.
Inland from the mangrove forests and coastal vegetation are the freshwater swamps, 25 600 km2 in extent (260 km2 reserved). Mitragyna ciliata, an important economic timber species, is typical of the swamp forests.
The mangrove and freshwater swamps, for which total wood production within Nigeria has been estimated as 609 630 000 kg /year present severe problems for exploitation because of their flooded conditions. There is no major exploitation as yet, other than for fuel and poles.
It is the high forest, which covers an area of 95 372 km2 (19 986 km2 reserved) which is the source of the bulk of the country's timber. The high forest is highly variable with regards to species composition and stocking and Hall (1977) suggested that the variation could be associated with soil differences. The forests of the central sub-group of the Ferralitic soils group are highly heterogeneous; but though they contain most of the major economic timber species, the greatest density of major economic timber stems occurs in the dry sub-group of the Ferruginous Tropical Soils Forests (Hall, 1977). In general the dry and semi-deciduous forests are richest in well-known commercially valuable timber trees which include Triplochiton scleroxylon, Afzelia africana and Mansonia altissima (Redhead, 1971).
Within the high forest zone of Nigeria alone approximately 26 000 hectares of forests are cleared annually and replanted with Entandrophragma spp., Gmelina arborea, Khaya spp., Lovoa trichilioides, Nauclea diderrichii, Terminalia ivorensis, T. superba, Tectona grandis and Triplochiton scleroxylon. Up to 1974, 3.83% of reserved high forest had been cleared and Umeh (1975) estimated that by 2 000 A.D. the exploitation rate of the high forest would be 129 000 hectares/year. In addition substantial areas are being cleared for urban and rural development, roads and food production.
At its northern margin the high forest gives way to the derived savannas, these in turn being replaced further north by the true savannas. The derived and true savannas, covering an extensive area of 850 000 km2 (75 000 km2 reserved), contain trees which can be used mainly for fuelwood, poles and rough timber. Species of some value are Pterocarpus erinaceus, Isoberlinia doka, Afrormosia laxiflora and Daniellia oliveri. Up to 1971 approximately 12 400 hectares of natural savanna had been clear-felled and replanted with Azadirachta indica, Cassia siamea, Acacia nilotica var. nilotica and species of Eucalyptus and Pinus.
IMPLICATIONS OF PRESENT LAND USE TRENDS
The yields of the natural forests are only about a tenth of what could be obtained from timber plantations. The trend is therefore towards the removal of the original cover followed by reforestation.
Removal of the forest cover may result in either a stable ecosystem or ecological imbalance. In the tropics generally soil fertility is tied up with the forest ecosystem and this fertility may be quickly lost if the forest cover is removed. The forest cover protects the soil from weathering and ensures regular water flow, controlling erosion.
Cultivation destroys many forested areas: for example, about 260 hectares are lost annually in farming encroachment in the Akure Forest Reserve alone. The attraction of the forested land for farmers is the higher topsoil fertility compared with the topsoil of recently cultivated ground. This reflects the effectiveness of the nutrient cycle in vegetation where woody species, which tend to be deeper-rooted than typical farm crops, predominate. However, it is clearly unsatisfactory for the existing forest estate to be reduced further as demand for timber continues to rise, and it is desirable that attention be given to alternative means of providing areas which farmers consider attractive. Where emphasis is on revenue, remunerative fast growing short rotation pulp species such as Gmelina could be grown independently of the activities of the Federal and State Forestry Services. Where emphasis is on food production, fast growing trees at an appropriate espacement could be combined with food crops in a similar situation, the aim being that the deeper-rooted trees will prevent excessive depletion of the nutrient reserves in the topsoil by returns of litter to the surface.
IMPLICATIONS OF EXCESSIVE EXPLOITATION
The intensive and extensive exploitation of natural forests has adverse effects on sources of seeds of many tree species for natural and artificial regeneration, because of the disperal characteristics and scarcity of individuals of some of these tree species. At the present rate of exploitation many Nigerian tree species, especially the rarer ones, are in serious danger of extinction.
The objectives of safeguarding forest tree genetic resources, which are of current or potential use, are to preserve them for their scientific, economic, educational and cultural use by present and future generations (Ola-Adams & Iyamabo, 1977). The preservation of the wide genetic diversity existing within the natural forests offers maximal scope for selection as new uses are found for wood. Moreover, criteria for selection in forest trees are likely to change with time. Such changes may occur in response to new technology, to different public acceptance, or in response to biological changes such as the constant search for forms of forest trees resistant to new strains of insects and pathogens.
CONSERVATION METHODS
There are many methods now available which could be used in conserving forest tree genetic resources and the appropriate method for any particular species will depend on the nature of the material and on the objective and scope of conservation envisaged (Frankel, 1970). Moreover, economic, technical and administrative factors will have to be taken into consideration. For example, in cases where populations of endangered species cannot be conserved in situ (e.g. individuals outside Forest Reserves or species heavily exploited) then it will be necessary to devise methods for ex situ conservation to ensure their perpetuation. Species for which it would clearly be appropriate to develop ex situ methods include Afrormosia elata, Entandrophragma utile and other mahoganies, Distemonanthus benthamianus and Gossweilerodendron balsamiferum. Nigerian experience gained in ex situ conservation of exotic pines and eucalypts (see Anon. 1976). may be applicable to some indigenous species also.
However, if endangered species of climax vegetation do not naturally regenerate after exploitation and cannot be cultivated easily with present knowledge, it is most likely that in situ conservation will prove the most immediately appropriate (Roche 1975). In situ conservation is feasible where pressure on existing forest reserves is light and this would be the case if more sources of timber were established outside forest reserves. At this stage it is necessary to consider the experience to date in Nigeria and the methods used so far.
EXPERIENCE TO DATE
In six methods of gene conservation there is already experience in Nigeria. Each is described briefly below:
(i) Strict Natural Reserves
Strict Natural Reserves are plant communities preserved in perpetuity in their natural state for their economic, scientific, education, cultural and aesthetic values.
The importance of preserving representative samples of diverse forest ecosystems in Nigeria was realised about 30 years ago. With the exception of very few species, most of the constituent species of the climax vegetation of the high forest zones are not domesticated and hence their continued existence depends on the continued integrity of the ecosystems in which they occur. Inviolate plots were therefore laid down in 1948 by the Nigerian Forest Department “to preserve typical pieces of Nigerian forests in an untouched state for posterity.” Since 1954 these areas have been designated “Strict Natural Reserves” by the Forestry Research Institute of Nigeria (formerly the Federal Department of Forest Research) in accordance with the 1933 London Convention on the preservation of the African Flora and Fauna. In 1968, Nigeria acceded to the OAU ‘African Convention for the Conservation of Nature and Natural Resources’ which defines Strict Natural Reserves (Article III Section 4a) and enjoins contracting States to “undertake the conservation of plant species and communities which are threatened and/or of special scientific or aesthetic value” There are now seven such reserves in Nigeria (Ola-Adams & Iyamabo, 1977).
(ii) Game Reserves
Game Reserves are areas set aside for the propagation, protection, conservation and management of flora and fauna.
Resource Management policies of many states apply to, among other different types of protected ecosystems, areas of land designated as Game Reserves. The preservation and management of the vegetation is a most important aspect of the management of any Game Reserve. Conservation of plant and animal species is the major reason for the setting up and subsequent management of Game Reserves. There are several gazetted Game Reserves in Nigeria, most in the savanna area. It is almost exclusively the savannas which are the object of protection, as a result of the protection considered necessary for the large game which occurs mainly in the savannas. There is, nevertheless, an equal need for Game Reserves in the high forest zone.
(iii) National Parks
A National Park is an area exclusively set aside for the propagation, protection, conservation and management of vegetation and wild animals as well as for the protection of sites, landscapes, or geological formations of particular scientific or aesthetic value, and for the benefit and enjoyment of the general public.
Plans are now at an advanced stage to constitute the present Borgu and Zugurma Game Reserves into Kainji Lake National Park. The formation of national parks calls for a great sacrifice on the part of the nation, as it may involve foregoing forever the advantages of exploiting the material resources of the area. While it is true that cultural, economic and recreational values are important factors in justifying the establishment or the maintenance of national parks, it is no less clear that one of the principal arguments in their favour is their conservational value, even though less directly appreciated by the general public. As new national parks are established, care must be taken to place under protection not only the most scenic landscapes or the areas with the most game, but also a wide variety of biotypes.
(iv) Botanic Gardens & Arboreta
Botanic Gardens and Arboreta are areas set aside for the propagation and conservation of plants for educational and scientific studies.
Attempts to establish Botanic Gardens have met with only partial success in this country. The old Lagos Botanic Station was destroyed by the development of the railway in 1896. In the early 50's four Botanic Gardens were destroyed at Lagos, Calabar, Olokemeji and Onitsha by the progress of urban and communications development. By 1958, there were only two Botanic Gardens left - the Victoria Botanic Gardens (now in Cameroun Republic) and the Botanic Gardens at the University College, Ibadan. A Federal Government proposal in the early 60's to establish Botanic Gardens at Enugu, Jos, Olokemeji and Onitsha, mainly for tourist attraction, did not materialise. Recently, however, most Universities and Forestry Departments have established Botanic Gardens and Arboreta for educational and research purposes, species trials and as repositories for endangered plants.
(v) Seed Storage
The storage of forest tree seed is a valuable method of conservation of the genetic resources of certain species in conditions that will guarantee the viability of the germ plasm for several years. The main objective in the storage of genetic stocks of seeds for long periods is to avoid the need for frequent rejuvenation, with its attendant costs and risks of genetic contamination.
The seed store at the Forestry Research Institute of Nigeria was constructed in 1959. Previously, short-lived seeds were sown soon after collection during the seed years, in nurseries which were usually irrigated, and in non-seed years wildings were also collected to supplement the seedling stock (Olatoye, 1968). Seeds are usually air-dried and placed in air-tight tins lined with polythene sheets and the seeds being stored are both indigenous and exotic. They are stored in tins in a cold room maintained at a temperature of 0°C to -5°C. At this low temperature it has proved possible to prolong the normal storage life of many indigenous forest tree species.
(vi) Clone banks
Forest tree genetic resources can also be conserved by methods of vegetative propagation, for example, grafting, cutting and marcotting. This technique is particularly appropriate for species which produce seed infrequently and for ‘recalci trants’ i.e. species of which the seed loses viability at low humidity (Roberts 1973).
Initial efforts to propagate some Nigerian forest trees by vegetative means have already met with some success (Jones, 1969, Okoro 1973, Howland 1975) and over ten years ago vegetative propagation techniques were used in establishing plantations of Terminalia ivorensis, T. superba, Nauclea diderichii and Triplochiton scleroxylon in various places.
Research efforts into the vegetative propagation of Triplochiton scleroxylon have shown that clonal propagation is possible, especially with young, juvenile materials (Okoro 1973, Howland 1975, Leakey et al 1975). Also attempts are being made to store cuttings of Triplochiton scleroxylon (Howland, pers. comm.).
ADDITIONAL FUTURE POSSIBILITIES
Research efforts in other parts of the world have indicated other methods which could also be of important value in conservation of forest tree genetic resources. These methods are briefly described below.
(i) Tissue and Meristem Culture
These techniques are particularly valuable for conserving species with recalcitrant seeds which cannot at present be conserved in seed banks and in cases of limited seed viability, unavailability of seed and ineffective vegetative techniques.
In some cases, there are good reasons to preserve a particular geno type or genotypes which have been well-tested and shown to contain desirable characteristics, such as resistance to disease or adaptation to climatic and day length factors. Although some of these plants can reproduce sexually, they are usually cultivated in the vegetative form. If seeds are used in cultivating such plants, the attendant selection environmental pressure, mutation, hybridisation and introgression can change the genotype. Tissue and meristem culture offer a means of surmounting this obstacle.
Another important advantage of these techniques is in making it possible to free material of infections or diseases. In plants in which the rate of growth is faster than that of the virus, the apical meristem can easily be cultured and this will be free of the infection.
The advantages of long-term storage of meristems - their small size, the ease with which control can be exercised over the rate of multiplication and the fact that they can easily be kept free from pathogens - could make tissue and meristem culture a very powerful tool in ex situ conservation.
Thus, although nothing is being done at present in this field in Nigeria, it is most desirable that the Forestry Research Institute of Nigeria should examine the possibility of using these techniques.
(ii) Pollen storage
Pollen is not being stored at present in Nigeria. There is nonetheless a need for short-term storage of pollen in hybridization experiments where plants flower at different times or where populations are geographically separated. It is particularly useful to be able to store pollen from season to season or for several years as a means of preserving plant genetic resources. The conditions necessary to maintain viability in pollen are similar to those required for seed storage, although the viability period for pollen is shorter. Pollen can be stored in liquid air or freeze-dried and sealed in glass capsules under a vacuum or in nitrogen gas (Harrington 1970). It is clearly worth ascertaining the conditions which will prolong for several years viability of the pollen of important tropical timber species. In fact, a proposal for short-term storage of pollen of Triplochiton scleroxylon is already under consideration.
LIMITATIONS
None of the methods outlined is completely free of drawbacks. The in situ methods are likely to prove most valuable in ensuring the conservation of species like the mahoganies. In such cases, Strict Natural Reserves, Game Reserves, National Parks, Botanic Gardens and Arboreta are the obvious means for conserving forest genetic resources, but there are many constraints. Generally, competition from other forms of land use and the complex land tenure system in the country constitute formidable constraints in establishing conservation areas large enough to cover all the variations of any particular ecotype. Even with the existing Strict Natural Reserves in Nigeria problems arise, because they have no legal status distinguishing them from the Forest and Game Reserves within which they are situated. For these Forest and Game Reserves in their entirety, certain rights are granted to the local people and timber contractors, e.g. right to fetch firewood, to collect other forest produce or to fell timber. When the Strict Natural Reserves have been established within these reserves, these rights have not been extinguished. Consequently, it is impossible to guarantee the inviolability of the existing Strict Natural Reserves under present circumstances.
Other major constraints are lack of sufficient funds, of professional and technical staff trained in gene conservation and resource management, and of adequate data on the biology, silvicultural characteristics and ecology of most indigenous forest trees.
Most of the Botanic Gardens and Arboreta have generally been established as collections of individual trees for limited scientific purposes, or for educational and recreational use, or for species trials. Too little emphasis is being placed on their conservational rôles as repositories of endangered plants. Endangered species could be much more widely incorporated, preferably represented by collections from different populations growing in a variety of habitats or locations throughout the natural range. Another serious disadvantage of Botanic Gardens and Arboreta is the past lack of proper and adequate documentation. There are cases of admixture of materials and labels. This, however, is an administrative weakness which could be remedied.
Major constraints of seed storage in Nigeria are especially the loss of viability within a short time by many indigenous forest tree species and the lack of adequate information on optimum temperature range and moisture content of many indigenous forest seeds (Olatoye 1968, Okoro & Dada 1971). There is a definite need for more seed stores in the country. Seed storage appears at present to have more potential value than pollen storage for the long term conservation of forest genetic resources.
The results of vegetative propagation of some indigenous forest tree species have been very encouraging. However, their use is restricted by stock/scion incompatibility in grafting some, and failure of cuttings of adult materials of some species to retain their leaves long enough for rooting to take place in others e.g. Triplochiton scleroxylon (Howland 1975).
Even with the methods not yet introduced, there are certain limitations which are likely to arise. The most serious disadvantage of tissue culture technique seems likely to be genetic instability. There may be nuclear changes such as small gene losses and gene mutations which may accumulate in culture (D'A mato 1975). Progress in this field is very rapid and it is probable that these limitations will soon be overcome. Presently, the genetic stability of the meristem can be successfully utilized by means of meristem culture. This technique is of the greatest value in vegetatively propagated plants, in which it permits the long-term conservation of a clone.
CONCLUSIONS AND RECOMMENDATIONS
Increasing demand for wood and competition from other forms of land use are placing considerable pressure on the available forest reserves in Nigeria.
In order to alleviate the exploitation pressure and farming encroachment on forest reserves, the State and Federal Governments should encourage cash cropping with trees and agri-silvicultural practices in areas outside forest reserves.
Many of the difficulties encountered in the formulation of methodology for the conservation of forest tree genetic resources result from the lack of sufficient financial assistance and of trained professional and technical staff, and inadequate data on the biology, silvicultural characteristics, ecology and seed storage conditions of many indigenous tree species.
Strict Natural Reserves, Game Reserves, National Parks, Botanic Gardens and Arboreta will serve as adequate means of conserving forest tree genetic resources, only if they fulfil certain conditions. Encouragement should be given to amending the policies adopted in these places so that conservation practices are intensified and improved. More conservation areas should be set up to cover typical samples of all major Nigerian vegetation types.
SUMMARY
Over-exploitation of natural forests, cultivation, urban and rural development programmes are considered to constitute the factors most responsible for the drain of the forest gene pool in Nigeria.
To counter this trend, suggestions are put forward for various possible methods of conserving forest tree genetic resources.
ACKNOWLEDGEMENT
I wish to express my gratitude to Dr. John B. Hall, Department of Forest Resources Management, University of Ibadan, for his comments and criticisms.
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1 Plant nomenclature follows Hutchinson & Dalziel.(1954–72)
